The present invention relates to the completion of subterranean well bores in a multilateral well system. More particularly, this invention relates to the sealing of junctions between lateral well bores and a parent well bore.
Operators seeking to produce hydrocarbons from subterranean formations often employ multilateral wells. Unlike conventional vertical wells, multilateral wells have a series of lateral well bores that branch off a primary well bore, which is typically drilled vertically. Although multilateral wells are often more expensive to drill and complete than conventional vertical wells, multilateral wells are generally more cost-effective overall, as they usually have greater productive capacity and higher recoverable reserves. Because fewer multilateral wells than conventional wells are needed to recover the same amount of hydrocarbons, overall drilling and capital expenses may be reduced. In addition to being cost-effective, multilateral wells are also an attractive choice in situations where it is necessary or desirable to reduce the amount of surface drilling operations, such as when environmental regulations impose drilling restrictions.
Although multilateral wells may offer advantages over conventional vertical wells, they may also involve greater complexity, which may pose additional challenges. One such challenge involves sealing junctions of well bores in the multilateral well system, for example, to prevent the infiltration of foreign matter into the multilateral well system. Infiltration of foreign matter can lead to a variety of problems. For example, if sediment around a junction is loose, it may enter the junction and may potentially impede or block flow within the well system, thereby potentially reducing or eliminating hydrocarbon production from a zone within the subterranean formation. Additionally, the infiltration of hydrocarbons through well bore junctions can lead to simultaneous production from multiple zones, possibly frustrating attempts to isolate and separately produce individual zones. Furthermore, penetration of water through well bore junctions into the hydrocarbon flow within the well system may necessitate additional costs, because the recovered hydrocarbons and any water mixed therein generally must be separated at the surface.
Conventional attempts to seal junctions of well bores have generally involved the placement of cement within an annulus between the formation and the junction of casing strings installed within each well bore. The cement forms a barrier to the formation, but generally does not penetrate into the formation to an appreciable distance, due to factors including, inter alia, the grain size of the cement and the permeability of the formation. Accordingly, a cement seal generally only seals the outer surface of a casing string to the face of the formation. Unfortunately, a cement seal may have a short useful life and is often unable to withstand the high temperatures and pressures typically found in subterranean well systems, along with stresses imparted by high fluid flows in the well bore, or by subsequent drilling operations conducted within the well system (e.g., the insertion and removal of a drilling string to and from the parent well bore, as additional lateral well bores are drilled at other locations lateral to the parent). Further complicating the use of cement to seal junctions is the fact that some form of mechanical isolation (e.g., a packer) is generally needed to prevent the accidental entry into the main well bore of the cement that is intended to seal the outer surface of the casing strings to the formation. Without the use of mechanical isolation, cement oftentimes will enter the main well bore, where it may need to be drilled out at additional expense, labor and time.
Other conventional attempts to seal junctions have involved the use of mechanical sealing devices, such as pieces of narrower piping that fit inside the well bore at a junction in such a manner as to seal the junction. These mechanical sealing devices may be difficult to install, and are also subject to the wear and tear caused by fluid flows and drilling.